A tonometer is an ophthalmological instrument for testing intraocular pressure. Such a test is commonly performed as part of a standard eye examination to detect the early stages of glaucoma. Glaucoma is characterized by an increase in pressure within the eye, which causes visual defects and ultimately may cause blindness. Because the patient seldom experiences any symptoms until major damage occurs, regular testing is essential to detect glaucoma in the early state before the retinal field is seriously diminished, and ocular nerve damage has occurred.
One type of tonometer is a “non-contact” tonometer; one common form of this type of tonometer is known as an “airpuff” tonometer. With this type of tonometer, a gas jet is directed toward the measurement area, which flattens the surface and a measure of reflected incident light yields a relative measurement of intraocular pressure. The non-contact devices are often large, since they include devices for positioning the patient's head, and rely heavily on the skill of the operator for accuracy. Patients sometimes complain of discomfort associated with the blast of air that must be delivered to obtain a measurement.
Another type of tonometer is a “contact” tonometer; one common form of this type is an applanation tonometer. Testing using a contact tonometer involves touching the tip of a probe directly onto the eye surface, and then pressing the tip against the surface toward the eye, thus indenting a portion of the surface of the eye, and determining the amount of force required to produce the given indentation. The resiliency of the surface and the internal pressure of the eyeball resist the flattening or indentation, and such force may be then converted to a measure of intraocular pressure. It is known that contact tonometers give more valid measurements of intraocular pressure, and provide more realizable diagnosis of early stages of glaucoma than the non-contact tonometers. Contact tonometers have been developed which are portable, weigh only a few ounces, and are thus convenient to use in any setting. These devices allow patients using mobility devices, or unable to make an office visit due to mobility, agoraphobia, claustrophobia, bedridden status, etc., to be tested, as the portable contact tonometer may be easily brought to their location. The small size of the instrument also provides a non-threatening glaucoma examination for apprehensive patients.
However, it is also known that a variety of disease pathogens can be found of the surface, and especially in the fluid film that covers the eye. These pathogens include those that related directly to the eye, such as conjunctivitis, and those which are systemic, such as immune system disorders. A disadvantage of contact tonometers is that they must touch the eye, and therefore, may pose a risk of transferring such pathogens from one patient to another, or from patient to health care provider. Various methods have been utilized to reduce the transmission risk, including sterilization of the probe tip between examinations. This method can produce eye irritation, and even damage if the disinfectant is not all removed from the surface. Sterile gloves must be used by the tester, or re-contamination may occur when reinserting the probe. Wiping of the probe tip surface may not be sufficient to remove all pathogens, and may also cause scratching of the probe's surface over time. For these reasons, a more preferred method has been to fit the probes, or contact tips, of contact tonometers with disposable covers, to prevent such transmission. Such covers can be disposed of after each measurement. They also help to protect the eye from injury or irritation. Such covers should be smooth in texture. They must have high mechanical strength so that thin layers may be applied to the probe tip and removed without tearing, and be a non porous barrier to liquids and microorganisms. Such covers are generally formed from materials such as natural latex rubber. However, it has been found that over time, persons can develop allergies to natural latex rubber, including mild contact irritation, delayed hypersensitivity, and systemic allergic reactions caused by antibodies to the proteins in the natural rubber. Persons who work in the health care industry are especially prone to development of such allergies because of multiple repeat exposures to natural latex.
It would be desirable to have a disposable cover formed of a material which would be hypoallergenic compared to natural latex rubber, but which would meet the other requirements of smoothness, strength, resistance to porosity of fluids or microorganisms, and non-irritating to the eye.
It has now been discovered that a hypoallergenic disposable tip cover for a contact tonometer may be formed from synthetic thin film materials such as silicone, polyurethane, polyisoprene and the like and that such cover will provide the required properties while also avoiding allergic reactions in patients and health care providers.